Oil and gas well drilling operations commonly use sensors deployed downhole as a part of the drill string to acquire data as the well bore is being drilled. This real-time data may provide information about the progress of the drilling operation or the earth formations surrounding the well bore. Significant benefit may be obtained by improved control of downhole sensors from the rig floor or from remote locations. For example, the ability to send commands to downhole sensors that selectively activate the sensors can conserve the battery life of the sensors and increase the amount of downhole time a sensor is useful.
Directional drilling operations are particularly enhanced by improved control. The ability to efficiently and reliably transmit commands from an operator to downhole drilling hardware may enhance the precision of the drilling operation. Downhole drilling hardware that, for example, deflects a portion of the drill string to steer the drilling tool is typically more effective when under tight control by an operator. The ability to continuously adjust the projected direction of the well path by sending commands to a steering tool may enable an operator to fine tune the projected well path based on substantially real-time survey data. In such applications, both accuracy and timeliness of data transmission are clearly advantageous.
Prior art communication techniques that rely on the rotation rate of the drill string to encode data are known. For example, Webster, in U.S. Pat. No. 5,603,386, discloses a method in which the absolute rotation rate of the drill string is utilized to encode data. While the Webster technique is serviceable, improvements could be made. For example, the optimum rotation rate of the drill string may vary within an operation, or from one operation to the next, depending on the type of drill bit being used and the strata being penetrated. As such, frequent reprogramming of the absolute rotation rates is sometimes required.
U.S. Patent Application 20050001737, to Baron et al., which is commonly assigned with the present application, discloses another technique for encoding data that also relies on the rotation rate of the drill string. The Baron technique advantageously overcomes the above-described difficulty, for example, by utilizing a difference between first and second rotation rates to encode data. While this approach is serviceable it may be improved upon for certain downhole applications. For example, drilling applications may be encountered in which the drill string sticks and/or slips in the borehole. This is a condition commonly referred to in the art as stick/slip, and is known to cause a non-uniform drill string rotation rate. In stick/slip situations, precise measurement of the drill string rotation rate sometimes becomes problematic. Therefore, there exists a need for improved techniques for communicating from the surface to a downhole tool.